Paramagnetic Rim Lesions in Multiple Sclerosis: Comparison of Visualization at 1.5-T and 3-T MRI

Abstract

Background: Multiple sclerosis (MS) is characterized by both acute and chronic intrathecal inflammation. A subset of MS lesions show paramagnetic rims on susceptibility-weighted MRI sequences, reflecting iron accumulation in microglia. These paramagnetic rim lesions (PRLs) have been proposed as a marker of compartmentalized smoldering disease. PRLs have been demonstrated at 7 T and, more recently, at 3 T. As susceptibility effects are weaker at lower field strength, it remains unclear if PRLs are visible at 1.5 T. Objective: To compare the visualization of PRLs using susceptibility-weighted imaging at 1.5-T and 3-T MRI in patients with MS. Methods: This retrospective study included 9 patients (5 women, 4 men; mean age, 46.8 years) with MS who underwent both 1.5-T and 3-T MRI using a comparable susceptibility-weighted sequence from the same manufacturer (GE SWAN). Lesions measuring >3 mm were annotated. Two reviewers independently assessed images at each field strength in separate sessions, classifying the annotated lesions based on susceptibility-weighted images as isointense, diffusely paramagnetic, or PRL. Discrepancies were discussed at consensus sessions including a third reviewer. Agreement was assessed using kappa coefficients. Results: Of 140 annotated lesions, based on the 3-T consensus readings, 115 (82%) were isointense, 16 (11%) were diffusely paramagnetic, and 9 (6%) were PRLs; based on the 1.5-T consensus readings, 115 (82%) were isointense, 14 (10%) were diffusely paramagnetic, and 11 (8%) were PRLs. Mean lesion diameter was 11.9 mm for PRLs, versus 6.4 mm for diffusely paramagnetic lesions (p=.006) and 7.8 mm for isointense lesions (p=.003). Interrater agreement for lesion classification as PRL was substantial at 1.5 T (kappa=0.65) and 3 T (kappa=0.70). Agreement for PRL was also substantial between the consensus readings at the two field strengths (kappa=0.79). Conclusion: We show comparable identification of PRLs at 1.5-T and 3-T MRI, with substantial interrater agreement at both field strengths and substantial consensus agreement between the field strengths. Clinical Impact: PRL may be an emerging marker of chronic neuroinflammation in MS. Their visibility at 1.5 T supports the translational potential of PRL identification to more widespread clinical settings, where 1.5-T scanners are prevalent.